Adrenaline controls how many times you can attack with a melee weapons of other weapons that do not use ammunition items.

Examples:

Weapons that use adrenaline: Marine Brass Knuckles, Predator Fistfight, and Alien Ram are a few.

Aliens and Predators are most dependent on Adrenaline unlike Marines who use weapons that require ammunition more.

Like blood, after being killed it will fully regenerate back to normal.

To heal yourself

• Marines will need to buy a Med Kit
• Aliens will use Hosts Devours
• Predators will use Steroids.

If you happen to run out of adrenaline, you will start to Bleed. While you are bleeding, you will lose blood over time. In order to stop the bleeding, you need to have at least 1 or more adrenaline.

If you get killed, along with loosing some of your Max Blood you will also loose some of your Max Adrenaline.

Your blood and adrenaline will never go as lower than (max adren level - (max adren level * 2)).
Example:

If you have 50 max adrenaline, you will never have more than -50 adrenaline missing. If you have 200 max adrenaline, you will never have more than -200 adrenaline missing.

Having it simply set back to 0 or 1 made it too easy for people to spam attack with large weapons, this way they are penalized for using weapons that are too strong for them. They have to replenish their negative adrenaline instead of just landing back at 1 and attacking again.